bladerf_get_gain_modes implemented inconsistently between bladeRF1 and bladeRF2.

[antiduh]

The documentation for bladerf_get_gain_modes says that you can pass in NULL so you can query the size of the mode list before you call it again:

https://www.nuand.com/libbladeRF-doc/v2.5.0/group___f_n___g_a_i_n.html#ga5350f1868a06dc92bef4ea0f07914f42

"This function may be called with NULL for modes to determine the number of gain modes supported."

And indeed this is how it's implemented for bladeRF1:

static int bladerf1_get_gain_modes(struct bladerf *dev,
                                   bladerf_channel ch,
                                   struct bladerf_gain_modes const **modes)
{
...
    if (modes != NULL) {
        *modes = mode_infos;
    }

However, bladeRF2 does not work this way. The first thing the function does is a null check, so the function stops immediately with an error code instead:

static int bladerf2_get_gain_modes(struct bladerf *dev,
                                   bladerf_channel ch,
                                   struct bladerf_gain_modes const **modes)
{
    NULL_CHECK(modes);